1. Field of the Invention
The present invention relates to the manufacturing of Schottky diodes, and especially of average-power Schottky diodes, capable of conducting a forward current with a low voltage drop, and to stand high reverse voltages with a small leakage current.
2. Discussion of the Related Art
Conventionally, such Schottky diodes are formed so that their junction/barrier is parallel to the main plane of the wafer in which they are formed, as illustrated in FIG. 1. It is spoken of a horizontal junction. The diode is formed from a heavily-doped silicon substrate 1 of a first conductivity type, here, type N, on which is formed a more lightly doped epitaxial layer 2. On the upper surface side is formed a Schottky diode between a metal, or more often a metal silicide 3, and epitaxial layer 2. To improve the breakdown voltage, the periphery of the Schottky junction is for example surrounded with a P-type ring 4. An insulating layer 5 is intended to ensure the necessary insulations. A metallization M1 is formed on the upper surface side, and a metallization M2 is formed on the lower surface side.
In such a Schottky diode, in the conductive state, the current flows from the upper surface to the lower surface of a silicon chip. It should however be noted that the barrier or Schottky junction is horizontal (parallel to the plane of the main diode surfaces).
Several improvements have been brought to the Schottky diodes, especially to reduce their forward voltage drop, by providing means for ensuring a depletion of the area corresponding to the substrate in the vicinity of the Schottky junction when the reverse voltage increases. Examples of such structures are illustrated in FIGS. 2, 3A, and 3B.
In FIG. 2, conductive fingers 10 surrounded with an insulator 11 penetrate into the substrate at regular intervals from the Schottky contact area, these fingers being in contact with the upper metallization. Such a Schottky diode structure, or MOS barrier Schottky diode, is currently designated as a TMBS (Trench MOS Barrier Schottky).
In FIG. 3A, P-type silicon fingers 20 in contact with the upper metallization penetrate into the substrate. Such a diode or Schottky-bipolar diode is currently called a JBS diode (for Junction Barrier Schottky).
FIG. 3B shows a variation with floating islets of the Schottky-bipolar diode of FIG. 3A, in which fingers 20 are divided into separate fingers 20a-20b coupled together by capacitive effect.
In all these known structures, the junction or Schottky barrier, possibly formed of several elements, is always horizontal for each of the considered elements.
Further, the applicant has described in unpublished French application 03/50985 of Dec. 5, 2003, inventor Jean-Luc Morand, semiconductor components with a vertical active junction.
The appended FIGS. 4 and 5 are FIGS. 3 and 4 of this patent application.
FIGS. 4 and 5 are cross-section and perspective views showing an example of cells of vertical junction diodes, with no metallization. A diode cell is formed of an interface between N and P regions 21 and 23, region 23 being in contact with a metallization 22, and region 21 being in contact with a metallization 24. All the metallizations of the same type as metallization 22 are connected to a same anode metallization, and all the metallizations of the same type as metallization 24 are connected to a same cathode metallization. Preferably, the anode and cathode metallizations are arranged on opposite surfaces of the component. An advantage of this structure for a power diode is that the junctions extend across the entire diode thickness and that that the junction surface area may be greater than in the case of horizontal junction diodes, in which the junction surface area is limited to the silicon chip surface area. In the embodiment of FIG. 4, the various diodes are arranged in parallel strips. In the embodiment of FIG. 5, the various diodes have circular structures. Other topologies may be adopted provided that the diodes have vertical junctions.
The structures of FIGS. 4 and 5 may easily be adapted to the forming of a Schottky diode with a vertical junction (or barrier) by replacing P-type region 23 with a metal, a metal silicide, or another material forming a Schottky barrier with an N region.